feat: implement v0.4.0 - Reports, Charts, Comparison, Dark Mode, E2E Testing

Backend (@backend-dev):
- Add ReportService with PDF/CSV generation (reportlab, pandas)
- Implement Report API endpoints (POST, GET, DELETE, download)
- Add ReportRepository and schemas
- Configure storage with auto-cleanup (30 days)
- Rate limiting: 10 downloads/minute
- Professional PDF templates with charts support

Frontend (@frontend-dev):
- Integrate Recharts for data visualization
- Add CostBreakdown, TimeSeries, ComparisonBar charts
- Implement scenario comparison page with multi-select
- Add dark/light mode toggle with ThemeProvider
- Create Reports page with generation form and list
- Add new UI components: checkbox, dialog, tabs, label, skeleton
- Implement useComparison and useReports hooks

QA (@qa-engineer):
- Setup Playwright E2E testing framework
- Create 7 test spec files with 94 test cases
- Add visual regression testing with baselines
- Configure multi-browser testing (Chrome, Firefox, WebKit)
- Add mobile responsive tests
- Create test fixtures and helpers
- Setup GitHub Actions CI workflow

Documentation (@spec-architect):
- Create detailed kanban-v0.4.0.md with 27 tasks
- Update progress.md with v0.4.0 tracking
- Create v0.4.0 planning prompt

Features:
✅ PDF/CSV Report Generation
✅ Interactive Charts (Pie, Area, Bar)
✅ Scenario Comparison (2-4 scenarios)
✅ Dark/Light Mode Toggle
✅ E2E Test Suite (94 tests)

Dependencies added:
- Backend: reportlab, pandas, slowapi
- Frontend: recharts, date-fns, @radix-ui/react-checkbox/dialog/tabs
- Testing: @playwright/test

27 tasks completed, 100% v0.4.0 implementation

frontend/src/components/layout/Header.tsx

@@ -1,18 +1,20 @@
 import { Link } from 'react-router-dom';
 import { Cloud } from 'lucide-react';
+import { ThemeToggle } from '@/components/ui/theme-toggle';
 
 export function Header() {
   return (
-    <header className="border-b bg-card">
+    <header className="border-b bg-card sticky top-0 z-50">
       <div className="flex h-16 items-center px-6">
         <Link to="/" className="flex items-center gap-2 font-bold text-xl">
           <Cloud className="h-6 w-6" />
           <span>mockupAWS</span>
         </Link>
         <div className="ml-auto flex items-center gap-4">
-          <span className="text-sm text-muted-foreground">
+          <span className="text-sm text-muted-foreground hidden sm:inline">
             AWS Cost Simulator
           </span>
+          <ThemeToggle />
         </div>
       </div>
     </header>